Place beets on large sheet of foil. Drizzle with 2 tbsp of the oil and 1 tsp each salt and pepper; wrap and roast on baking sheet until fork tender, about 1 hour. Open foil and let beets sit until cool enough to handle. Rub skins off using hands or paper towel; finely dice.
Meanwhile, bring 5 litres water to boil in large pot. Add 1 tbsp of the salt and quinoa, and boil on high for 11 minutes. Drain in fine-mesh sieve, then rinse under cold water. Let cool.
In large bowl, combine beets, quinoa, red pepper, green onions, orange segments, orange juice, mint leaves, the remaining oil and the remaining salt. Gently stir in mixed lettuce until evenly coated.